3.0 Application Hints

To get the expected results when measuring temperature

with an integrated circuit temperature sensor like the LM74,

it is important to understand that the sensor measures its

own die temperature. For the LM74, the best thermal path

between the die and the outside world is through the LM74’s

pins. In the SO-8 package all the pins on the LM74 will have

an equal effect on the die temperature. Because the pins

represent a good thermal path to the LM74 die, the LM74 will

provide an accurate measurement of the temperature of the

printed circuit board on which it is mounted. There is a less

efficient thermal path between the plastic package and the

LM74 die. If the ambient air temperature is significantly

different from the printed circuit board temperature, it will

have a small effect on the measured temperature.

In probe-type applications, the LM74 can be mounted inside

a sealed-end metal tube, and can then be dipped into a bath

or screwed into a threaded hole in a tank. As with any IC, the

LM74 and accompanying wiring and circuits must be kept

insulated and dry, to avoid leakage and corrosion. This is

especially true if the circuit may operate at cold temperatures

where condensation can occur. Printed-circuit coatings and

varnishes such as Humiseal and epoxy paints or dips are

often used to insure that moisture cannot corrode the LM74

or its connections.

3.1 micro SMD LIGHT SENSITIVITY

The LM74 in the micro SMD package should not be exposed

to ultraviolet light. The micro SMD package does not com-

pletely encapsulate the LM74 die in epoxy. Exposing the

LM74 micro SMD package to bright sunlight will not imme-

diatly cause a change in the output reading. Our experi-

ments show that directly exposing the circuit side (bump

side) of the die to high intensity (

centered at a wavelength of 254nm, for greater than 20

minutes will deprogram the EEPROM cells in the LM74.

Since the EEPROM is used for storing calibration coeffi-

cients, the LM74 will function but the temperature accuracy

will no longer be as specified. Light can penetrate through

the side of the package as well, so exposure to ultra violet

radiation is not recommended even after mounting.

4.0 Typical Applications

10090920

FIGURE 8. Temperature monitor using Intel 196 processor

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